1. Field of the Invention
The present invention generally relates to an image forming apparatus such as an electrophotographic printer, facsimile, and copier.
2. Description of the Related Art
Conventionally, as a device for fixing an unfixed toner image transferred on a sheet such as paper in these image forming apparatuses, a heat roller fixing device has been known and widely used. In the heat roller fixing device, while the sheet having the unfixed toner image is sandwiched and conveyed, the sheet is heated and pressurized by a fixing nip part formed of a fixing roller incorporating a halogen heater and the like and a pressure roller that pressurizes the fixing roller, so that the toner image is fixed.
Moreover, a belt fixing device has also been known. In the belt fixing device, an endless fixing belt is stretched by a heating roller incorporating a halogen heater and the like and a fixing roller, and a fixing nip part formed of the fixing belt and a pressure roller that pressurizes the fixing roller interposing the fixing belt heats and pressurizes the sheet having the unfixed toner image while sandwiching and conveying the sheet, so that the toner image is fixed.
By these fixing methods, a toner image fused on a sheet contacts a fixing member such as the fixing roller and fixing belt. Therefore, surfaces of these fixing members are coated with a fluororesin having superior releasability, and a separation claw for separating the sheet from the fixing member is used. When using the separation claw, however, a claw mark or claw scar is easily formed on the surface of the fixing member because the separation claw contacts the fixing member. As a result, there is a problem in that a striation is generated on an output image.
In the case of a monochrome image forming apparatus, in general, a fixing roller is formed of a metal roller whose surface is coated with Teflon (registered trademark). Therefore, a scar is not easily formed on the fixing roller even when contacted by the separation claw. Accordingly, the fixing roller has a long life. In the case of a color image forming apparatus, however, a surface layer of a fixing roller is formed by coating a silicone rubber with fluorine in order to improve color development (generally, about several tens of microns of PFA (Tetra fluoro ethylene perfluoro alkyl vinyl ether copolymer) tube is used). Alternatively, the surface layer of the fixing roller is formed by applying oil to a silicone rubber. The surface layer with such a structure is soft and vulnerable.
When the surface layer gets a scar, a scar like a striation appears on a fixed image. Therefore, in many color image forming apparatuses today, a non-contact separation unit is used instead of using a contact unit such as the separation claw that contacts the fixing member, to separate a sheet from the fixing member without contact. In the separation without contact, a sheet on which toner is fixed is easily wound around a fixing member when an adhesion force between the toner and fixing member is high. In this case, a winding jam occurs, in which a sheet passes through a gap while tightly contacting the fixing member. In the color image forming apparatus, in particular, plural toner layers are stacked on a sheet. Therefore, a winding jam easily occurs due to an adhesion force increased by the stacked toner layers. In particular, a sheet having a small blank space at a leading end part, on which no toner is attached; and a sheet on which a solid image such as a photograph is formed easily cause the winding jam due to their adhesion forces.
At present, the following sheet separating methods are used for separating sheets in the color image forming apparatus.
(1) A non-contact separation plate method, in which a separation plate extending in parallel with a longitudinal direction and a width direction of a fixing roller and a fixing belt is provided so that a fine gap (about 0.2 mm to 1.0 mm) is formed between the fixing roller and fixing belt.
(2) A non-contact separation claw method, in which separation claws are arranged at a predetermined interval in a longitudinal direction of a fixing roller and a fixing belt so that a fine gap (about 0.2 mm to 1.0 mm) is formed between the fixing roller and fixing belt.
(3) A self-stripping method, by which a sheet is naturally peeled off due to an elasticity of the sheet and an elasticity of curved parts of a fixing roller and a fixing belt.
In any of these methods, however, there is a gap between the separation plate or separation claws and the fixing roller and fixing belt. Thus, when passing a thin sheet, a sheet having a small blank space at a leading end, or a solid image such as a photograph, the sheet passes through the gap while the sheet is tightly contacting the fixing roller or fixing belt. Therefore, a winding jam of the sheet occurs, or a jam is caused by the sheet hitting the separation plate or separation claw.
In view of the above-mentioned, there has been suggested and used a unit for blowing air onto a sheet at a sheet separation position to separate the sheet from the fixing roller and fixing belt, in order to reinforce the non-contact separation unit. In this manner, in an image forming apparatus provided with a non-contact separation unit that separates a sheet by blowing compressed air from an air nozzle to the sheet at a sheet separation position without damaging a fixing member such as a fixing roller and a fixing belt, a compressed air generating unit (hereinafter arbitrarily referred to as a compressor) and a pneumatic piping extending from the compressor to the air nozzle for controlling and circulating the compressed air are provided.
Each of six to ten air nozzles provided in the image forming apparatus has a cross sectional area with a diameter φ=1 mm or less. The air nozzles blow air for about 100 msec at a moment when a leading end of a sheet leaves a fixing nip part. Thus, a flow volume of the compressed air is small. Accordingly, the pneumatic piping is constituted by arranging a tank with an appropriate capacity (for example, about 1 liter) for accumulating the compressed air and the like, a small compressor, and a relief valve. This constitution is preferable because a structure is compact, and there are few pressure variations at the time of ejecting air. With this constitution, however, excess air has always been discharged from the relief valve while the compressor is operating, and wasted without being used. Further, since a noise generated when the excess air is discharged is noisy, a silencer is required to be provided at a discharge outlet for noise control, which has been a factor to increase the cost.
In Patent Document 1, in order to prevent a decrease of a temperature at a surface of a fixing member caused by air blown onto the fixing member by a residual pressure in an air pipe line facing the fixing member even after a sheet is separated from the fixing member by an air blow, an additional air pipe line that deviates the fixing member is provided. By letting the residual pressure out from this air pipe line, an air blow onto the fixing member caused by the residual pressure is reduced. However, piping of the additional air pipe line and utilization of air are not described in detail in Patent Document 1.    [Patent Document 1]    Japanese Patent Application Publication No. 2008-102408